docs(chat): record streaming-ux progress, block task 3 on batch three, align DESIGN.md

DESIGN.md gains the streaming-states table, the thinking-collapse and
label-swap motion rows, the 17px body correction and the single live
marker rule; task 3 is registered in BLOCKED.md until the home split
batch three lands; PROGRESS lists every touched assertion with its
former value.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JUei7K13cYxLHE3Axe4A45

frontend/src/lib/stream-frame-buffer.ts

@@ -104,7 +104,6 @@ export function createStreamFrameBuffer<Meta>(
   let answerBacklog = 0;
   let thinkingBacklog = 0;
   let meta = options.initialMeta;
-  let dirty = false;
   let disposed = false;
   let frameHandle: number | null = null;
   let timeoutHandle: number | null = null;
@@ -121,7 +120,6 @@ export function createStreamFrameBuffer<Meta>(
   };
 
   const emit = (settled: boolean) => {
-    dirty = false;
     options.flush({
       answer: releasedAnswer,
       thinking: releasedThinking,
@@ -173,12 +171,10 @@ export function createStreamFrameBuffer<Meta>(
     setMeta(next) {
       if (disposed) return;
       meta = typeof next === "function" ? (next as (current: Meta) => Meta)(meta) : next;
-      dirty = true;
       schedule();
     },
     touch() {
       if (disposed) return;
-      dirty = true;
       schedule();
     },
     settle() {
@@ -198,7 +194,6 @@ export function createStreamFrameBuffer<Meta>(
       releasedThinking = "";
       answerBacklog = 0;
       thinkingBacklog = 0;
-      dirty = false;
       if (nextMeta !== undefined) meta = nextMeta;
     },
     dispose() {